In the cryopreparation of objects, in particular biological objects, which are to be subjected to subsequent examination under a microscope or an electron microscope, there is a need for the temperature of the gas atmosphere in the interior of the cooling chamber of a microtome and more particularly an ultramicrotome to be set to a precise value. In that connection, the aim is for it to be possible for the temperature to be set within a relatively wide range going from about -40.degree. C. to about -160.degree. C. While it is relatively easy to set the temperature of the gas atmosphere in the cooling chamber in the vicinity of the lower temperature limit, that is to say around the above-mentioned figure of -160.degree. C., it has been found that problems arise in adjusting the temperature of the gas atmosphere in the middle and upper ranges of temperature in such a way that there is a uniform temperature over the entire space within the cooling chamber of the microtome.
However, many objects require a processing temperature, for example when carrying out a cutting operation thereon, which is in the middle or higher temperature ranges because such objects become brittle at low temperature and do not give a clean cut. The processing temperature can be adjusted by controllable heating resistors in the object holder disposed in the cooling chamber for holding the object to be processed, and in the processing tool itself. However the drain of heat from the cutting region of the processing tool and from the tip of the object being processed, into the cold gas atmosphere in the cooling chamber, can be so great that the cutting edge and the tip of the object are always at a temperature which is close to the temperature of the gas atmosphere.
In an endeavour to deal with that problem, it is possible to heat not only the processing tool and the object holder, but also the gaseous cryogenic agent which fills the interior of the cooling chamber, in accordance with PCT-publication No. WO 88/02851. That is achieved by means of a gas heating plate which is arranged over the floor of the cooling chamber interior in such a way that the gaseous cryogenic agent which passes into the interior of the cooling chamber by issuing from the delivery opening of a feed line opening into the interior of the cooling chamber, can flow away over the heating plate and its temperature is raised as a result.
It has been found however that it is not possible to achieve a satisfactory temperature distribution configuration in respect of the gas atmosphere in the cooling chamber in that way, apparently because only a thin layer of the gaseous cryogenic agent, as it flows over the heating plate, is raised to the desired temperature and rises within the cooling chamber, without effecting uniform temperature distribution therein. In addition, because of the existing temperature gradient troublesome turbulence phenomena may even occur in the gas atmosphere in the cooling chamber, and such turbulence not only interferes with the normal cutting operation to be carried out in the cooling chamber, but it can also permit moist air to penetrate into the interior of the cooling chamber. That moist air then results in frosting on the processing tool and the object so that satisfactory processing of the object is no longer possible under those circumstances.